In electrical engineering it is frequently desirable to be able to distribute a voltage to any selected one of a plurality of circuits, the voltage in question being supplied on an input conductor and being distributed to a desired output conductor by manual switching or by relay operation. It is well known, for example, to tap an input line at successive points therealong and connect the taps through independent switching contacts to independent output circuits, as in the simple case of several doorbells energized from a common battery or transformer through independent push buttons.
When the signal to be distributed is more complex, as for example comprising a train of essentially square wave pulses having high repetition frequencies and short rise and fall times, the problem becomes more complicated if signal distortion is to be avoided. Here it is desirable to use feed lines having appropriate known characteristic impedances. Concentric cables of various known characteristic impedances are known, and relays are also known which operate successfully to switch signals of the type described without distortion, but such relays are expensive and the process of connecting a common input conductor to numerous relays using coaxial cables is intricate and time consuming, as is any repair procedure later found necessary. Relays for use with coaxial cable also take up more space, since they must be so located that their terminals are accessible for attaching the cable.
Another form of feedline useable for distribution of signals made up of digital pulse trains comprises a strip transmission line or "stripline". This feedline arrangement places the feedline in the center of a sandwich structure, the feedline taking the shape of a thin, ribbon conductor. The sandwich, of thickness "b" consists of two dielectric sheets, each of thickness b/2.
The ribbon conductor of width W runs between the two dielectric sheets. The outside of each dielectric sheet is totally clad with copper to form a groundplane. Z.sub.0, the characteristic impedance of such a line, when unloaded, is determined by the dielectric constant and thickness of the dielectric sandwich, b, and the width of the ribbon conductor, W.
Inexpensive, compact relays have been designed for use with printed circuit boards. These relays have a construction characterized by a "footprint" or planar arrangement of connection points when the relay is secured to the strip-line conductor within the stripline circuit board. The conductors of the board must be provided with solder pads for connection to the relay contacts. The solder pads have lumped capacitances which make the loaded characteristic impedances of the striplines different from their known unloaded values. It is also known that the relay itself, when tapping into such a line, introduces a further lumped capacitance and causes more energy reflection, so that altogether the signal wave form is considerably degraded in its pulse width, rise time, and fall time.